Rotary electrical coupling for microwave heating apparatus

ABSTRACT

Disclosed is an electrical connection system for a microwave oven including a chamber defining a housing, a source for transmitting microwave energy into the chamber, a turntable rotatably mounted within the chamber and defining stations for receiving articles to be heated and a drive mechanism for producing rotation of the turntable. A control circuit controls energization of the source in response to signal outputs from temperature responsive sensors positioned in the heating stations. The rotary coupling includes a stationary portion mounted on the housing and having stationary contacts connected to the control circuit and a rotary portion mounted on the turntable and having rotary contacts connected to the sensors and slidably engaging the stationary contacts during rotational movement of the turntable.

BACKGROUND OF THE INVENTION

This invention relates generally to microwave apparatus for safelyheating a batch of distinct articles and, more specifically, to such anapparatus for thawing the frozen parenteral solution content of plasticbags.

The addition of medication to parenteral solutions is a widely acceptedclinical practice. Although many intravenous additives presently areprepared in patient care areas by nursing staff, there is an increasingpressure on hospitals to establish pharmacy admixture programs tominimize medication errors and the risk of microbial contamination. Tooptimize the use of available manpower, some pharmacies have adopted thepractice of freezing admixtures.

It is known that several antibiotics in clinical use are stable forextended periods of time when frozen in large volume parenteralsolutions. Indeed, freezing of antibiotic solutions often can prolongstability and suspend growth of essentially any microbial contaminant.Studies have shown that antibiotic admixtures are unharmed for up to 30days if frozen in 100 or 50 ml. admixture container bags. However, theseveral hours required for room temperature thawing of these units oftenmake this technique unsuitable in the hospital. Some improvement inpreparation efficiency can be achieved by thawing the bags in warm waterbaths, a technique that is widely practiced in blood banks. Warm waterthawing, however, still consumes significant time in addition torequiring labor intensive procedures for both cleaning baths to reducecontamination levels and avoiding contamination by touch during poolingoperations.

As a solution to these problems, microwave ovens have been proposed asvehicles for thawing frozen admixtures prior to intravenousadministration. The resultant rapid thawing would allow centralizedadmixture programs to operate a 24-hour service without the need for apharmacy to be ready to make admixtures at all times. Other attributeswould include the opportunity to take advantage of the efficiencyaccompanying large scale manufacturing techniques. Especially importantwould be the possibility of introducing pre-release quality control onmanufactured lots, presently not possible for admixtures prepared byconventional methods where the maximum permissible refrigerated storageis 24 hours.

Although the efficiency of microwave heating presents many advantages,prior microwave ovens do not satisfy all requirements for safely thawingindividually bagged units of parenteral substances on a scale necessaryfor practical applications. Even the most sophisticated timing andtemperature sensing controls available with existing microwave ovenscannot prevent under all conditions the overheating of individual unitsduring batch thawing thereof. For example, the inadvertent combination,in a given batch, of units exhibiting substantially different volumes orchemical characteristics can cause non-uniform temperature increases andresultant overheating of specific units. Similar consequences can derivefrom inadvertently including a thawed or partially thawed bag with afully frozen batch. In addition to possibly degrading the therapeuticeffectiveness of its content, overheating can cause rupture of anindividual bag. Under certain conditions the explosive discharge from aruptured bag can injure attendant personnel and in all cases must becleaned from the equipment surface on which it collects.

The object of this invention, therefore, is to provide microwave ovenapparatus that can safely, efficiently and simultaneously thaw thefrozen parenteral mixture content of multiple plastic bags.

SUMMARY OF THE INVENTION

The invention comprises an electrical connection system for a microwaveoven including a chamber defining a housing, a source for transmittingmicrowave energy into the chamber, a turntable rotatably mounted withinthe chamber and defining stations for receiving articles to be heatedand a drive mechanism for producing rotation of the turntable. A controlcircuit controls energization of the source in response to signaloutputs from temperature responsive sensors positioned in the heatingstations. The rotary coupling includes a stationary portion mounted onthe housing and having stationary contacts connected to the controlcircuit and a rotary portion mounted on the turntable and having rotarycontacts connected to the sensors and slidably engaging the stationarycontacts during rotational movement of the turntable. The inventionfacilitates the retrieval of signal information from temperature sensorsmoving within a microwave oven chamber.

A featured embodiment of the invention includes an electrical signalmultiplexer mounted within the oven chamber and having an inputconnected to receive output signals from all the sensors and an outputconnected to the rotary contacts and adapted to sequentially transmitthereto all of the output signals in a predetermined sequence. Themultiplexer increases the signal capacity of the rotary couplingassembly.

According to one feature of the invention, the multiplexer is retainedin an electrically conductive can, an output surface portion of which iscovered with a dielectric material and an electrically conductive skirtadjacent thereto. Microwave energy is shielded by the conductive can anddissipated by the capacitive block formed by the can, the dielectricmaterial and the conductive skirt thereby preventing interference withthe multiplexer signals.

According to another feature of the invention, the rotary couplingportion comprises an insulator column extending through a wall portionof the housing and having an outer surface supporting the rotarycontacts that comprise longitudinally spaced apart rings and thestationary coupling portion comprises an insulator block supporting thestationary contacts and having a cylindrical opening receiving theinsulator column. The stationary contacts include a plurality of springcontacts arranged in pairs, the spring contacts forming each pairstraddling and engaging a different one of the ring contacts and beingelectrically interconnected. This arrangement establishes efficientelectrical coupling and the redundant pairs of spring contacts preventloss of signal in response to the inadvertent misalignment or vibrationof the various individual components.

According to still another feature of the invention, the rotaryturntable includes a centrally located post, a shaft coaxially andslidably received thereby, means keying the shaft to the post forrotation therewith, and a release mechanism operable to permit relativeaxial movement between the post and the shaft. After actuation of therelease mechanism the insulator column can be withdrawn from theinsulator block permitting removal of the entire turntable assembly fromthe oven chamber. Reassembly of the components is simplified by aconically shaped terminal portion on the insulator column that engagesthe pairs of spring contacts during movement of the column into theinsulator block.

DESCRIPTION OF THE DRAWINGS

These and other objects and features of the invention will become moreapparent upon a perusal of the following description taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic perspective view of a microwave oven according tothe invention;

FIG. 2 is a schematic front view, partially in cross section, of oneheating station of a turntable illustrated in FIG. 1;

FIG. 3 is a schematic side view, partially in cross section of theheating station illustrated in FIG. 2;

FIG. 4 is a schematic cross-sectional view of a temperature sensorillustrated in FIGS. 2 and 3;

FIG. 5 is a schematic elevational view of a rotary electrical couplingshown in FIG. 1;

FIG. 6 is a schematic cross-sectional view taken along the lines 6--6 ofFIG. 5;

FIG. 7 is a schematic cross-sectional view taken along the lines 7--7 ofFIG. 6;

FIG. 8 is a schematic cross-sectional view through a center column ofthe rotary coupling shown in FIGS. 5-7;

FIG. 9 is a schematic view illustrating assembly details of theturntable and rotary coupling shown in FIG. 1; and

FIG. 10 is a schematic block circuit diagram of an electrical controlcircuit for the microwave oven shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a microwave heating apparatus 11 constructed inaccordance with the present invention. A housing 12 defines an ovenchamber 13, access to which is provided by a door 14'. Rotatablysupported within the chamber 13 is an article supporting turntableassembly 14 that is coupled to a rotary drive mechanism 15. A microwavesource 16 transmits into the oven chamber 13 radiant energy for heatingarticles retained therein. Disposed on a control panel 17 of the housing12 is a display 18, a plurality of oven control buttons 19 and a programcard insert slot 21. Interconnected with the control panel 17, themicrowave source 16, and the drive mechanism 15 and a dc power supply 22is an oven control printed circuit 23. The foregoing portions of themicrowave unit 11 are used in commercially available microwave ovens.

Retained by the turntable 14 is a control station 24 and a plurality ofannularly distributed auxiliary stations 26 each adapted to receive anarticle to be heated within the chamber 13. Positioned within thecontrol station 25 is a temperature responsive control sensor 27 while atemperature responsive safety sensor 28 is positioned in each of theauxiliary stations 26. A safety control circuit 31 is electricallycoupled to an alarm horn 32, an alarm light 33, a reset switch button 34and the oven control circuit 23. Electrical signals from the sensors 27,28, respectively, are fed out of the chamber 13 to the oven controlcircuit 23 and the safety control circuit 31 via a rotary electricalcoupling assembly 35. The heating stations 25, 26; the sensors 27, 28;the rotary coupling assembly 35 and the safety control circuit 31 aredescribed in detail below.

Referring now to FIGS. 2 and 3, there are shown in greater detail thecontrol station 25 and the control sensor 27. The auxiliary stations 26and the safety sensors 28, respectively, are structurally identical tothe control station 25 and the control sensor 27 and therefore will notbe further described. Forming the control station 25 is a tray 37 havinga lower end retained by a base portion 36 of the turntable 14 and anupper end retained by a top portion 38 thereof. The control sensor 27includes a horizontally oriented terminal portion 41 juxtaposed acentral portion of the tray 37. Extending from the upper portion of thetray 37 is a hook 39 that retains an article 40 to be heated. Inpreferred use of the invention, the article 40 would consist, forexample, of a flexible plastic bag (as shown) containing a frozensubstance to be administered intravenously. When positioned on the hook39 the bag 40 overlays and engages the terminal portion 41 of the sensor27 so as to be in good thermal contact therewith. As shown in FIG. 3,the terminal portion 41 of the sensor 27 has an elongated cross sectionso as to establish a planar surface substantially parallel to and inintimate contact with the bag 40 thereby improving heat transfertherebetween. As also shown in FIG. 3, the tray 37 is supported at asmall angle of, for example, six degrees with respect to vertical. Thatangle can be increased if increased rates of heating are desired forfewer stations.

As shown in FIG. 4, the control sensor 27 includes an elongated tubecomprising the horizontal portion 41 and a vertical portion 42 thatextends out of a sealed metal can 43 that also accommodates the safetysensors 28 (FIG. 5). Extending through the tube 41, 42 is a cable 44composed of a pair of insulated signal wires 45, 46, a braidedconductive shield 47 and an insulator cover 48. The outer ends of thesignal wires 45, 46 are connected to the terminals of a thermistor 49while the inner ends thereof are connected to an electrical circuitretained within the can 43 as described below. Enclosing the outer endsof the wires 45, 46 and the thermistor 49 is an electrical insulatortube 51. The volume between the braided shield 47 and the horizontaltube portion 41 is filled with potting material such as silver epoxyhaving high thermal conductivity so as to establish good heat transferbetween the tube portion 41 and the thermistor 49.

Heat transfer between the bag 40 (FIG. 2) and the thermistor 49 ismaximized by the high thermal conductivity of the tube portion 41 andthe potting compound 52. However, direct microwave energy heating of thesensor 27 is minimized by the tube 41, 42 that functions as a microwaveenergy shield. Preferably, the tube 41, 42 is formed of a highlyconductive material such as brass and plated to an extremely fine finishwith a very low resistive material such as silver. In this way surfaceconductivity is maximized and microwave heating of the sensor 27minimized because of the skin effect phenomenon. In addition, the lengthof the sensor 27 is selected so as to establish therefor a detunedantenna condition at the microwave frequencies transmitted by the source16. Such detuning further limits microwave heating of the sensor 27.

During assembly of the sensor 27, the end of the braided shield 47 iswithdrawn to expose the ends of the signal wires 45, 46 which areattached to opposite terminals of the thermistor 49. The insulator tube51 then is positioned over the thermistor 49 and covered with thepreviously withdrawn portion of the braided shield 47. Thus prepared,the thermistor assembly is inserted through the open end of the tubeportion 42 until reaching the position within the tube portion 41illustrated in FIG. 4. An electrically and thermally conductive pottingcompound 52 such as silver epoxy then is faced through an opening 55 tocompletely fill the tube portion 41. In addition to providing a mass fortransferring heat between the bag 43 and the thermistor 49, the material52 seals the opening 55 to establish complete microwave shielding of thecomponents within the tube 41.

Referring now to FIGS. 5-8, there are shown further details of therotary feedthrough assembly 35. A cylindrical insulator block 57 ismounted on a wall option 58 of the housing 12 that closes the top of theoven chamber 13. Extending axially through the cylindrical block 57 isan opening 59 that accommodates a column assembly 61 the lower end ofwhich extends through the wall portion 58 and is attached to the can 43.A plurality of spaced apart, radial slots 62 enter one longitudinalsurface portion of the block 57. Radially projecting into an oppositelongitudinal surface portion of the block 57 are a plurality of spacedapart slots 63. Although only one slot 63 is shown in FIG. 6, it shouldbe understood that the block 57 contains a plurality of slots 63, oneradially aligned with each of the slots 62. Retained by a screw in eachof the slots 62 is a resilient conductive blade 64 having an outerportion projecting beyond the outer surface of the block 57 and an innercontact 65 spring loaded against the column 61. Similarly, each of theslots 63 retains a resilient conductive blade 66 having an outer portionextending out of the block 57 and an inner contact 67 spring loadedagainst the column 61. The portions of each aligned pair of the blades64, 66 that extend out of the block 57 are electrically connected byinsulated conductors 71. In addition, the outer terminal portion of eachof the blades 64 is connected to a different insulated signal wire 72-76(FIG. 5). round shields 77 for all of the signal wires 72-76 areelectrically connected to a bus wire 78 extending longitudinally of theblock 57 and having opposite ends inserted into receiving openingstherein.

Referring now to FIG. 8, there are shown details of the central column61 which includes a stacked plurality of identical insulator discs 81straddled by a conically shaped insulator 82 and an elongatedcylindrical insulator 83. Circumferential recesses are formed alongupper and lower edges of the discs 81, a lower edge of the conicallyshaped insulator 82 and an upper edge of the elongated insulator 83.Adjacent pairs of these recesses form annular channels each conformingto and receiving a ring contact 84-88 and a metal cylinder 91encapsulates the lower portion of the elongated insulator 83. Each ofthe contact rings 84-88 is positioned so as to be engaged by a differentpair of the spring loaded contacts 65, 67. Press fitted through centralapertures in the discs 81-83 is an alignment rod 90 that also extendsthrough an upper wall 89 of the can 43 and is secured thereto by a nut90'. Also formed in each of the discs 81-83 is a longitudinal slot, allof which are aligned to provide a longitudinal channel 93 through thecolumn assembly 61. Extending through the channel 93 and an opening 92in the wall portion 89 are a plurality of electrical leads 94-98, oneelectrically connected to each of the contact rings 84-88. Opposite endsof the leads 94-98 are connected to electrical circuitry in the can 43as described below. Thus, the leads 94-98 are individually connected,respectively, to the signal wires 72-76 (FIG. 5) via the ring contacts84-88 and the blade contacts 64, 66.

Referring again to FIGS. 5 and 7, there is depicted in greater detailthe can 43 which, as described below, houses electronic circuitryconnected between the sensors 27, 28 and the rotary coupling 35. Asshown, the can 43 is supported by a shaft 95' and in turn supports thecentral column 61. To limit the entry of microwave energy, the walls 96'of the can 43 are preferably formed of a good electrically conductivematerial such as brass and plated to a fine finish with a highlyconductive material such as silver to establish minimum surfaceresistance. In addition, the side walls 96 of the can 43 are coveredwith a dielectric material 97', for example Teflon tape, that isseparated by a small air gap from a cylindrical metallic skirt 98'secured to the wall portion 58 by screws 99. Together, the dielectriccoating 97' and straddling conductive skirt 98 and side walls 96 form acapacitive block that serves to dissipate microwave energy preventingits entry into the can 43. Further isolation is provided by an annular,energy absorbing disc 101 positioned between the can 43 and the wallportion 58 and an annular, energy absorbing disc 102 positioned betweenthe wall portion 58 and the rotary coupling 35. The discs 101 and 102are formed of a suitable microwave energy absorbing material such asEccosorb (SFT-2.5) sold by Emmerson and Cummings of Canton, Mass.

Illustrated in FIG. 9 is the manner in which the can 43 and thecomponents associated therewith are supported on the turntable 14 withinthe chamber 13. The hollow shaft 95' supporting the can 43 is slidablyreceived over a post 103 extending centrally from the turntable 14.Secured to the post 103 and extending through a base portion of anL-shaped slot 104 in the shaft 95' is a pin 105 that prevents relativevertical movement therebetween. A second pin 106 is removably receivedby a hole in the post 103 and also extends through the base portion ofthe slot 104. When in position, the pin 106 prevents relative rotationalmovement between the shaft 95' and the column 103 that would align thepin 105 with the upright portion of the slot 104. However, upon removalof the pin 106, such relative movement can occur allowing the shaft 95'to move downwardly on the post 103 as the pin 105 is received by theupright portion of the slot 104. The relative vertical movement betweenthe post 103 and the shaft 95 is sufficient to permit withdrawal of thesupport can 43 and column 61 from both the rotary coupling assembly 35and the skirt member 98 (FIG. 7). Thus, those components together withthe turntable 14 can be removed from the oven chamber 13 through thedoor 14. During reassembly, the conically shaped insulator 82facilitates centering of the column 61 during entry into the couplingassembly 35 and also functions to gently separate the spring loadedcontacts 65 and 67 (FIG. 6) as the column 61 enters the central opening59 in the insulator block 57.

FIG. 10 depicts in block diagram form an electrical circuit 111 forcontrolling operation of the microwave oven 11. Included in the controlcircuit 111 are a multiplexer 112 and the safety control circuit 31 thatincludes a comparator section 114 and a signal conditioner and latchsection 115. A variable resistor 116 provides an adjustable referencevoltage for the comparator 114. The multiplexer 112 is housed in the can43 (FIGS. 5 and 7) and receives on lines 45 and 46 the output of thecontrol thermistor 49 in the control sensor 27. Also received by themultiplexer 112 are the outputs of thermistors R_(t1), R_(t2) . . .R_(tn) retained by the safety sensors 28 (FIG. 1). Connecting themultiplexer 112 to the safety circuit 31 is the rotary coupling 35. Thesafety circuit 31 receives power from the supply 22 and is connected toenergize the horn 32 and the light 33 and to be released from a latchedcondition by the reset button switch 34. Receiving signals from both thesignal conditioner section 115 and the rotary coupling 35 is the ovencontrol circuit 23 that in turn controls energization of the microwavesource 16.

OPERATION

During operation of the microwave unit 11, articles to be heated arepositioned in the stations 25, 26 within the oven chamber 13. Inspecific applications, the articles would consist of flexible plasticbags 40 filled with a frozen substance suitable after thawing forintravenous administration. Although any of the stations 26 can remainunoccupied by a frozen bag, a bag should always be positioned in thecontrol station 25 before each thawing cycle. For that reason, thecontrol station 25 is clearly distinguished from the auxiliary stations26 by a label 121. In addition, the filled volumes of all bags in agiven run should be substantially equal and their contents shouldpossess substantially similar thawing characteristics. After theturntable 14 has been filled with the number of bags required, the door14 is closed, a suitable program card is inserted into the slot 21 andthe oven control buttons 19 are operated in a conventional manner toenergize the microwave source 16 and begin a thawing cycle. As thethawing process proceeds, the thermistor retained by each sensor 27, 28in an occupied station exhibits a resistance proportional to thetemperature of the bag with which it is in thermal contact. Conversely,the thermistors retained by sensors in unoccupied stations reflect theambient temperature in that station. The temperature indicating signalfrom the control sensor 27 on the lines 45, 46 is fed through the can 43directly to the leads 97, 98 via the rotary coupling 35 and output lines75, 76 to the oven control circuit 23. However, the individual outputsof the safety sensors 28 are received by the multiplexer 112 thatcontinuously transmits those signals over the leads 94, 95 in apredetermined sequence. The multiplexed signals are fed through therotary coupling 35 and over the output lines 72, 73 to the comparatorsection 114. DC supply voltage is received by the multiplexer 112 fromthe safety circuit 31 via the line 74, the rotary coupling 35 and thelead 96. A common for both the dc supply and the output of the controlsensor 27 is provided by the line 75, the lead 97 and the rotarycoupling 35. The oven control circuit 23 is programmed to monitor athawing operation and control energization of the source 16 in responseto signal information from the control sensor 27. When that signalinformation indicates that substantially the entire content of the bagin the control station 25 has reached a fully thawed temperaturethreshold of, for example, 25° C., the oven circuit 23 terminatesoperations. At that time, the bags retained in any of the auxiliarystations 26 will have reached substantially the same temperature becauseof the even distribution of microwave energy established by the rotatingturntable 14. At the conclusion of a defined cycle, the door 14 can beopened and the thawed bags removed and used as desired.

Assume, however, that a bag in one of the auxiliary stations 26inadvertently reaches a predetermined maximum safe temperature thresholdof, for example, 50° C. Temperature levels above that threshold couldeither damage the bag's fluid content or induce bursting thereof. Such asituation can result from various circumstances such as, for example, ifan auxiliary station 26 retains at the initiation of a thawing cycle abag that is thawed or partially thawed, a bag of lesser volume than thebag retained in the control station 25, or a bag filled with a substancehaving a substantially different thermal characteristic than the bagretained in the control station 25. Upon reaching the predeterminedunsafe temperature, the safety sensor 28 associated with that bagproduces via the multiplexer 112; the data wires 94, 95; the rotarycoupling 35 and the output leads 72, 73 a signal voltage below areference voltage established by the adjustable resistor 116 within thecomparator 114. In response thereto, the comparator section 114 andsignal conditioner and latch section 115 produce an output to the ovencircuit 23 inducing deenergization of the microwave source 16. Inaddition, the signal conditioner section 115 energizes the horn 32 andthe light 33 providing both audible and visible indications that thepredetermined safe temperature limit has been exceeded. Thawingoperations can be resumed only after the overheated article is removed,the overheated sensor cools below the threshold point and the resetbutton 34 is manually operated to eliminate the latch condition existingin the circuit section 115.

An important feature of the invention prevents the safety sensors 28 inunoccupied auxiliary stations 26 from reaching a temperature that wouldcause oven shutdown. This feature permits use of the oven unit 11 withthe turntable 14 filled to less than full capacity. Referring again toFIG. 4, the mass provided by thermally conductive potting material 52functions as a heat sink that disperses the limited amount of heat thatis generated by microwave energy present on the surface of the shieldportion 41. Consequently, the mass 52 in an unoccupied station 26 andthe thermistor retained thereby will not reach oven shutdown temperatureduring normal operations. Conversely, in the absence of a frozen bag inthe control station 25, the mass 52 in the control sensor 27 is selectedso as to be heated by microwave energy to a temperature above thatrequired to normally terminate a thawing operation. Thus, in the eventthat a frozen bag is not positioned in the control station 25, thecontrol sensor 27 will quickly reach a temperature that will induce theoven control circuit 23 to react as if a regular thawing cycle has beencompleted. A normal cycle can proceed only after an operator hasdiscovered the problem and positioned a frozen bag in the controlstation 25.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. For example only, althougha prefered use of the invention has been described, it will be obviousthat the oven 11 can be used to safely heat batches of articles otherthan those specifically mentioned. It is to be understood, therefore,that the invention can be practiced otherwise than as specificallydescribed.

What is claimed is:
 1. Microwave oven apparatus comprising:housing meansdefining an oven chamber; door means providing access to said chamber;source means energizable to transmit microwave energy into said chamber;support means rotatably mounted within said chamber and defining aplurality of heating stations for receiving articles to be heated; drivemeans energizable to produce rotation of said support means; temperatureresponsive sensor means mounted on said support means for rotationtherewith, said sensor means comprising a plurality of temperatureresponsive sensors each positioned and arranged to sense the temperatureof an article retained in a different one of said stations; controlcircuit means mounted outside said chamber and adapted to controlenergization of said source means in response to signal output from saidsensor means; a rotary coupling assembly electrically connecting saidcircuit means to said sensor means, said coupling assembly comprising asubstantially stationary portion mounted on said housing means andhaving substantially stationary contact means connected to said controlcircuit means and a rotary portion mounted on said support means andhaving rotary contact means connected to said sensor means, said rotarycontact means slidably engaging said stationary contact means duringrotational movement of said support means; and multiplexer means mountedon said support means within said chamber and having an input connectedto receive output signals from all of said sensors, said multiplexermeans having an output connected to said rotary contact means andadapted to sequentially transmit thereto all of said output signals in apredetermined sequence.
 2. Apparatus according to claim 1 including anelectrically conductive can completely enclosing said multiplexer meansso as to provide microwave energy shielding therefor.
 3. Apparatusaccording to claim 2 including dielectric means covering an outersurface portion of said can and an electrically conductive skirt meansdisposed adjacent to said dielectric means; said can, said dielectricmeans, and said skirt means forming a capacitive block that dissipatesmicrowave energy transmitted by said source.
 4. Apparatus according toclaim 2 wherein said rotary contact means comprises a plurality ofelectrically isolated rotary contacts each connected to a differentoutput terminal of said multiplexer, and said stationary contact meanscomprises a plurality of electrically isolated stationary contacts, eachslidably engaging one of said rotary contacts.
 5. Apparatus according toclaim 4 wherein said rotary portion comprises a cylindrical insulatorcolumn extending through a wall portion of said housing and with anouter surface supporting said rotary contacts that compriselongitudinally spaced apart ring contacts; and said stationary portioncomprises an insulator block supporting said stationary contacts andhaving a cylindrical opening receiving said insulator column. 6.Apparatus according to claim 5 wherein said stationary contacts comprisea plurality of spring contacts arranged in pairs, the spring contactsforming each said pair straddling and engaging a different one of saidring contacts and being electrically connected by conductor means. 7.Apparatus according to claim 6 wherein said rotary support comprises acentrally located post, a shaft coaxially and slidably received by saidpost, a means keying said shaft to said post for rotational movementtherewith, and release means operable to permit relative axial movementbetween said post and said shaft so as to allow withdrawal of saidcolumn from said insulator block.
 8. Apparatus according to claim 7wherein said column comprises a conically shaped terminal portionadapted to engage said pairs of spring contacts during movement of saidcolumn into said insulator block.
 9. Microwave oven apparatuscomprising:housing means defining an oven chamber; door means providingaccess to said chamber; source means energizable to transmit microwaveenergy into said chamber; support means rotatably mounted within saidchamber and defining a plurality of heating stations for receivingarticles to be heated; drive means energizable to produce rotation ofsaid support means; temperature responsive sensor means mounted on saidsupport means for rotation therewith, said sensor means comprising aplurality of temperature responsive sensors each positioned and arrangedto sense the temperature of an article retained in a different one ofsaid stations; control circuit means mounted outside said chamber andadapted to control energization of said source means in response tosignal output from said sensor means; and a rotary coupling assemblyelectrically connecting said circuit means to said sensor means, saidcoupling assembly comprising a substantially stationary portion mountedon said housing means and having a plurality of substantially stationarycontact connected to said control circuit means and a rotary portionmounted on said support means and having a plurality of rotary contactseach connected to one of said sensor means and slidably engaging one ofsaid stationary contacts during rotational movement of said supportmeans and wherein said rotary portion comprises a cylindrical insulatorcolumn extending through a wall portion of said housing and with anouter surface supporting said rotary contacts that compriselongitudinally spaced apart ring contacts; and said stationary portioncomprises an insulator block supporting said stationary contacts andhaving a cylindrical opening receiving said insulator column. 10.Apparatus according to claim 9 wherein said stationary contacts comprisea plurality of spring contacts arranged in pairs, the spring contactsforming each said pair straddling and engaging a different one of saidring contacts and being electrically connected by conductor means. 11.Apparatus according to claim 9 wherein said rotary support comprises acentrally located post, a shaft coaxially and slidably received by saidpost, a means keying said shaft to said post for rotational movementtherewith, release means operable to permit relative axial movementbetween said post and said shaft so as to allow withdrawal of saidcolumn from said insulator block.
 12. Apparatus according to claim 11wherein said column comprises a conically shaped terminal portionadapted to engage said pairs of spring contacts during movement of saidcolumn into said insulator block.